Variable area exhaust nozzles for gas turbine engines are old in the art and come in many different configurations. Some are simply two-dimensional convergent nozzles, as represented by U.S. Pat. No. 4,013,226 wherein a single flap is actuated by one or more hydraulic actuators and is pivotable about rollers and movable, along a curved track which cooperates with the rollers such that the nozzle area may be varied or even blocked. Commonly owned U.S. Pat. No. 3,354,649 shows a three-dimensional convergent nozzle comprising a plurality of flaps pivotally attached to an axially translatable unison ring and which are moved along curved cam tracks by hydraulic actuators.
Convergent/divergent variable area three-dimensional nozzles are also well known. Representative of such nozzles are U.S. Pat. Nos. 3,792,815; 4,141,501; 4,196,856; and 4,245,787. Each of these patents describe nozzles having convergent flaps, divergent flaps, and external nozzle flaps. Except for the '856 patent, in each case either the convergent or external flaps are pivotally connected to fixed engine structure. In the '856 patent the upstream ends of the external flaps are axially translatable independently of the motion of the convergent and divergent flaps by actuator means which operates independently from the actuation system which moves the convergent and divergent flaps. While this allows considerable freedom of motion, the requirement of a second actuation system is undesirable in view of its added weight, cost, and complexity.
It is commonly an objective in nozzle designs to minimize the required actuator loads to operate the nozzle. Thus the nozzle flaps and linkage arrangements are sometimes designated such that the pressure loads acting on certain portions of the nozzle, and which tend to open the nozzle, are partially or even completely balanced by pressure loads which tend to close the nozzle. For example, in aforementioned U.S. Pat. No. 3,792,815 an additional upstream balance flap extends forwardly from the forward end of the convergent nozzle flaps. Pressure loads on these balance flaps tend to rotate the convergent flaps to a closed position to counter loads on the nozzle assembly which are trying to force the nozzle open. While the balancing feature of the '185 patent works well, a simpler, less expensive means for balancing the nozzle assembly is highly desirable.